This invention relates to an electrochemical secondary element with a positive electrode having an active material which includes a lithium intercalating chalcogen compound of a transition metal, a negative electrode having an active material which includes a lithium intercalating carbon product, and a non-aqueous electrolyte.
Until recently, the utility of rechargeable lithium cells was limited in many fields because of the lack of cycling capability and high self-discharge. While weight and size of the cells would be greatly reduced, and thereby well adapted to current consuming devices, lithium cells principally remained as primary elements because their ability to be repeatedly recharged proved to be unsatisfactory.
A significant breakthrough in rechargeable lithium electrodes arose from the discovery of electron-conducting framework materials which are capable of intercalating and deintercalating lithium ions in their host lattice during alternate charging and discharging. The lithium electrode is replaced by an electrode formed of a Li-intercalation compound, but with the difference that the electrode reactions of the lithium electrode proceed largely reversibly via the host lattice-matrix, free from undesired side effects such as sludge or dendrite formation.
Japanese patent application (JP-A) 81-94788 discloses a lithium button cell in which graphite in the form of a pressed powder tablet constitutes the matrix of the negative lithium electrode. The tablet-shaped positive counter electrode consists of a transition metal oxide or sulfide. These compounds crystallize, for example, in layered lattices with relatively weak binding forces in the direction perpendicular to the layers, which permits a reversible insertion of lithium ions between such layers.
Thus, the charge/discharge operation of such a cell is carried out through the alternate charging (doping) of one electrode matrix with Li.sup.+ ions and the discharging (undoping) of the other electrode matrix of Li.sup.+ ions, which serve as the true electrochemically active ion species.
However, according to U.S. Pat. No. 4,668,595, graphite storage compounds are not stable in conjunction with organic electrolytes whereas, in place of graphite, certain carbon products which are obtained from organic polymer compounds through a delayed coking process are much more suitable as a carbon matrix. In referring to their disordered microcrystalline structure, such carbon products are also called turbostratic carbon materials (see F. Levy, "Intercalated Layered Materials", D. Reidel, Dorndrecht, Germany, 1979).
In combination with lithium intercalating transition metal oxides as the material of the positive electrode, it is possible to produce (using the above-named carbon electrolytes as well as a non-aqueous electrolyte based on a lithium salt) rechargeable lithium batteries in a charged state which exhibit rest potentials up to 4.5 volts. The construction of the cell takes place with uncharged electrodes, i.e. with Li-free carbon and a metal oxide doped with Li.
In another known button cell disclosed in U.S. Pat. No. Re. 33,306, the positive electrode is a Li-free metal oxide, i.e charged, in the initial assembly state. The negative electrode is formed as a carbon pressed body in close conductive contact with a lithium disk, and is also charged. By allowing the cell to stand, the carbon material becomes doped through self-discharge of the lithium in situ with Li.sup.+ ions, so that upon subsequent charging of the cell when placed in operation, a lithium-containing carbon intercalate is formed.